1. Field of the Invention
The present invention relates to a system for controlling the air-fuel ratio in an internal combustion engine provided with fuel injectors, the system being operated by a feedback control to maintain a constant air-fuel ratio under various engine operating conditions.
2. Description of the Prior Art
Japanese Unexamined Patent Publication (Kokai) No. 56-6034 discloses an air-fuel ratio control system for an internal combustion engine in which a basic fuel-requirement signal is generated in response to certain engine operating parameters, including engine temperature, which indicate the fuel requirements of the engine while running in a stable condition. This prior art also discloses a means for detecting the operating condition of the engine during a transient state and indicating a required increase in an engine output, and means for generating a signal for intensifying the air-fuel ratio correction in accordance with the sensed engine temperature and the sensed operating condition of the engine during the transient state. This air-fuel ratio correction signal has an initial value determined by the engine temperature and the sensed engine transient state, and this value is increased toward 1.0 at a speed determined by the temperature of the engine. The prior art also includes a means for supplying fuel to the engine in accordance with the above-mentioned basic signal and the intensifying air-fuel ratio correction signal, so that a required amount of fuel is supplied to the engine at both the stable running condition and the transient state. This system can maintain an optimum or predetermined air-fuel ratio not only in the stable running condition but also in the transient state of the engine, thus obtaining an optimum engine operation.
A disadvantage of this prior art system is that it is not provided with a means for compensating the changes in the characteristics of the engine which occur after the engine has been in operation for a prolonged period, such as oil or carbon deposits in the valve clearance, in the injector nozzle, or at the back of the intake valve. These deposits originate mainly from oil or carbon residues caused by combustion. The characteristics of an engine also change when different blends of gasoline are used. Thus, a system having no means of compensating for such changes in characteristics is disadvantaged in that the air-fuel mixture will often become lean during acceleration, causing a rough acceleration and an inferior drivability, when an inferior blend of gasoline is used or when the engine characteristics are changed after a prolonged period of operation. Also, when the correct blend of gasoline is used, the fuel consumption efficiency is decreased, or the amount of toxic emissions is increased, by a rich air-fuel mixture during acceleration.
An attempt was made to solve this problem in the prior art by controlling the correction factor during acceleration or deceleration in accordance with the engine warm-up condition. However, this was insufficient to cope with the above-mentioned changes in the air-fuel ratio caused by the changes in the engine characteristics, such as carbon or oil deposits on the valve plate of the intake pipe or an incorrect blend of gasoline.